1. Field of the Invention
The invention relates broadly to the field of RF transmitters for Wireless Communications applications and in particular the issues associated with RF power amplifier linearization.
2. Description of Related Art
As Wireless Communications standards evolve to support greater spectral efficiency and bandwidth they make use of advanced modulation approaches such as CDMA and OFDM which have high Peak to Average power ratios, typically in the range of 10 dB, i.e. the peak RF power is 10 dB greater than the average power. This requires the transmitter RF power amplifier to have sufficient peak power capability to support the high power peaks while operating on average at much lower power levels.
RF power amplifiers have a non-linear RF gain and phase characteristic verses RF output power which becomes increasingly non-linear as the RF output power approaches the PA maximum saturated output power (Psat). Power amplifier non-linearity creates distortion in the transmitter output which results in degraded signal quality and spectral re-growth. Wireless communications standards typically define transmitter spectral emission requirements and metrics of signal quality such as EVM or equivalent. Power amplifier non-linearity may result in the transmitter failing to meet these standard defined spectral emissions or signal quality requirements. Two possible approaches may resolve the non-linearity problem: 1) Operate the RF power amplifier at sufficiently low average RF power relative to the PA saturated power capability to ensure the PA is exercised over a range where the non-linearity is acceptable. 2) Use some form of linearization approach to correct the power amplifier non-linearity. The disadvantages of the first approach include poor efficiency; class AB power amplifier efficiency degrades as the average output power is backed-off further from Psat, and the additional cost incurred in providing a power amplifier with higher peak power capability. This invention details a new approach to power amplifier linearization that incorporates pre-distortion.
Pre-distortion corrects the power amplifier non-linearity by applying a non-linear function (pre-distorting) the input signal to the power amplifier such that the cascade of the pre-distortion non-linear function and the power amplifier non-linearity has an overall linear characteristic, or at least is closer to linear than the original power amplifier characteristic. The goal in a wireless communications system is to achieve sufficient linearity to comply with the standard defined spectral mask and signal quality metrics while operating the PA at a lower back-off level than could otherwise be achieved, therefore optimizing the transmitter efficiency and peak power capability.
The new approach proposed is based on digital pre-distortion meaning the non-linear pre-distortion function is applied in the digital domain then applied to a digital to analogue converter and up-converted to RF and then applied to the power amplifier input. The optimisation of the pre-distortion function is performed by sampling the power amplifier output, down-converting and applying to an analogue to digital converter. This digital signal is then further processed by a pre-distorter estimator in order to optimise the pre-distorter non-linear function to minimize the power amplifier distortion.
In broad terms digital pre-distortion approaches can be split into two classes, memory-less and those which compensate for memory effects. RF power amplifiers, and particularly when operated with wide bandwidth signals, exhibit a non-linear distortion characteristic which is not time invariant, but rather is dependent upon the history of the previous output power. Pre-distortion techniques which seek to model the non time invariant nature of the power amplifier distortion are classed as memory methods, those which do not as memory-less methods.
In order to improve power amplifier efficiency it is desirable to adopt power amplifier architectures such as 2-way and 3-way Doherty. These power amplifier architectures may offer significantly higher power amplifier efficiency, but experimentally they may exhibit higher levels of non-linear distortion compared to class AB architectures, and may present more significant memory effects when supporting instantaneous bandwidth requirements commonly required by modern wireless communications standards. Commercial factors for improving power amplifier efficiency include a strong desire to improve system efficiency to both minimize the end users energy costs and reduce thermal dissipation to allow for convection cooling deployment scenarios as opposed to forced air cooling, again saving cost. Therefore it is highly desirable to implement the invention disclosed herein of a pre-distortion system which corrects for memory effects in order to support the use of high efficiency power amplifier architectures such as Doherty.
Digital pre-distortion approaches including those which address memory effects can be split into techniques based on a polynomial pre-distorter model or approaches using table based approaches.
U.S. Pat. No. 6,903,604 describes a good example of a polynomial based approach which also addresses memory effects. Typically with high efficiency Doherty amplifiers, as discussed previously, they exhibit a highly non-linear response with discontinuities apparent over a wide dynamic range which to model accurately would require a relatively high order polynomial. The computational complexity and therefore time required in a typical commercial application to optimise the polynomial model is adversely affected as polynomial order is increased. It is also widely understood that numerical stability is degraded with increased polynomial order. An alternative approach based on a multi-dimensional table structure is detailed in U.S. Pat. No. 6,798,843. This approach avoids the issues associated with the higher order polynomials but requires significant computational resource.